This invention relates to a method of recovering uranium from wet process phosphoric acid obtained by acid decomposition of phosphate rock of natural occurrence by utilizing gypsum as a medium for the recovery of uranium.
Phosphate rocks of natural occurrence generally contain about 100-200 ppm of uranium. In the wet process manufacture of phosphoric acid by wet decomposition of phosphate rock with a mixed acid consisting of sulfuric acid and recycled phosphoric acid, most of uranium contained in the phosphate rock transfers into the phosphoric acid solution obtained as the liquid component of a gypsum slurry. Since wet process phosphoric acid is manufactured in an enormous quantity, recovery of uranium from wet process phosphoric acid solution has long been tried although the uranium content in the solution is not so high.
For industrially recovering uranium from wet process phosphoric acid, various kinds of recovering methods such as the solvent extraction method, ion-exchange method, precipitation method and adsorption method have been proposed until now.
Particularly the solvent extraction method has already been industrialized in several countries, but this method is disadvantageous in some respects. Firstly, the cost of equipment becomes high because there is the need of refining the phosphoric acid by a pretreatment in order to prevent formation of sludge at the stage of extraction. Besides, the solvent for the extraction is an expensive one, and therefore the recovery must be carried out by complicated operations in order to avoid the loss of the expensive solvent.
The ion-exchange method also requires a certain pretreatment of the phosphoric acid solution. Furthermore, in this method it is necessary to considerably lower the concentration of the phosphoric acid solution to be introduced into the ion-exchange column from usual concentrations of phosphoric acid produced by the wet process. By reason of such inconveniences this method has not yet widely been industrialized. Neither the precipitation method nor the adsorption method has been put into industrial practice mainly because of expensiveness of the precipitating agent or the adsorbing agent and inevitableness of a considerable loss of the expensive agent.
Japanese Patent Application No. 55(1980)-102409 discloses that hemihydrate gypsum in a phosphoric acid solution exhibits surprisingly different affinities for tetravalent ions and hexavalent ions of uranium and consequentially captures the tetravalent ions with a selectivity factor of nearly 100%, and proposes to enhance the uranium concentration in a wet process phosphoric acid solution by performing the wet process such that hemihydrate gypsum is formed in the presence of an oxidizing agent in the acid solution to render uranium dissolved in the solution entirely hexavalent. However, this Japanese patent application gives no new teaching about the method of recovering uranium from the phosphoric acid solution prepared by the improved process.
Japanese Patent Application Primary Publication No. 55(1980)-144419 proposes to incorporate a solvent extraction process for the recovery of uranium in a so-called hemihydrate-dihydrate type wet process for the manufacture of phosphoric acid, in which process calcium sulfate is intermediately formed as hemihydrate and subsequently converted to dihydrate. The uranium recovering process according to this proposal is inseparable from the process of preparing phosphoric acid and is applicable only to the hemihydrate-dihydrate process. In other words, this proposal cannot be applied to the other types of wet process for the manufacture of phosphoric acid, such as the dihydrate process, anhydride process, hemihydrate process and dihydrate-hemihydrate process. Besides, a uranium-containing solution obtained by the proposed treatment using gypsum as a recovering medium still contains large amounts of P.sub.2 O.sub.5 and H.sub.2 SO.sub.4, and therefore the recovery of uranium from this solution must be accomplished by a solvent extraction method under restricted conditions in order to avoid the loss of P.sub.2 O.sub.5. This solvent extraction method is also disadvantageous in using an expensive solvent for the extraction and in requiring costly apparatus.